1. Field of the Invention
The present invention relates to a turning mirror having one or more reflective surfaces, which is applied to allow laser beam to scan in a laser beam printer and to a method for manufacturing such turning mirror.
2. Description of the Related Art
Recently more and more widely used as a printing terminal for information related machines are laser beam printers, which advantageously offer excellent quality, high-speed, and low mechanical noise printing on plain papers. Such a laser beam printer comprises a turning mirror which reflects laser light emitted by a laser device so that the reflected laser light may be directed onto a recording drum for recording.
FIG. 11 is a partial view of a typical construction of a conventional laser beam printer. As seen from FIG. 11, the laser light emitted by a semiconductor laser device 1 which is driven by data signal is converged onto a recording drum 3 by a plurality of condenser lenses 2. A turning mirror 4 is disposed between these condenser lenses. The turning mirror 4 rotates at a high speed, causing the laser beam to scan, and consequently allowing a light spot 5 to scan over the recording drum 3. Meanwhile, the recording drum 4 slowly rotates in its subscanning movement which is combined with the already described main scanning movement, thereby allowing the light spot to form a two-dimensional latent image on the recording drum 3.
In such a scanning optical system described above, a turning mirror requires a high degree of accuracy in order to form the image of any given input picture on the recording drum 3. Specifically, to achieve a fixed scanning period and excellent printing quality, the inclination accuracy of the turning mirror with respect to its reference plane must be accurate to few tens of second in angle. Furthermore, the reflective surfaces of the turning mirror must meet the following rigorous requirements: a flatness accurate to a fraction of .lambda. (.lambda. period of helium-neon laser light, 632.8 nm), a surface irregularity of a few hundredths .mu.m or less, and a reflectivity of 85% or more. To satisfy such accuracy requirements, in a conventional manner, pure aluminum bars have been machined to form structures, reflective surfaces on which are then put to a lathe for mirror finishing so that the reflective surfaces are accurate to required accuracy.
In a second conventional method, a turning mirror has been formed of entirely synthetic resin. In this method, an injection molding technique has been employed. Typically, polycarbonate has been employed as a synthetic resin. Reflective film is formed in deposition process onto surfaces to be used as a reflective surface.
In the case where a turning mirror is applied as a polarizer, a motor is coupled to the turning mirror by means of shrinkage fit or by means of gluing process accurately onto the rotating shaft of the motor, which is manufactured up to a good square, or accurately onto the hub mounted on the motor rotating shift.
In the above-described conventional arrangement and method of production, poor productivity results in the first convention example as a result of using mirror finish machining. The mirror finishing machining also needs an expensive numerical control cutting machine, pushing up manufacturing costs. In a single mirror-surface type turning mirror of which mirror surface is angled at approximately 45 degrees with respect to the axis of rotation, mirror finish machining cannot be performed in a stacked manner to a plurality of mirror surfaces at a time, because such a manner is expected to degrade accuracy in inclination. Thus, productivity is even more lowered.
In the second conventional method in which injection molding technique is applied to resin, all molds used must be of metallic one to withstand high pressure in molding process. To achieve required flatness and required surface irregularity on a polygon mirror by means of metal molds, material stability of the metal molds is critical. Mold materials which are capable of accurate molding normally exhibit poor processability, and thus the production of the molds are difficult process. Since injection molding is performed under high temperature and high pressure conditions, it is normally difficult to assure required accuracy on molded structures. Particularly, an irregularly configured structure such as a single mirror-surface type turning mirror presents difficulty in molding process. In the case of a multi-plane type turning mirror, assuring required inclination accuracy of each plane in injection molding process is even more difficult.
Since the resulting mirror of a molded structure is not symmetrical unlike a spherical surface, an ambient temperature rise easily gives rise to mirror surface deformation, which is sufficient enough to deteriorate the mirror beyond the flatness requirement. Thus, implementation of such method has been difficult.
Furthermore, when a reflector mirror fabricated by injection molding is used as a polarizer, some degree of degradation is introduced in accuracy of the reflector mirror when the reflector mirror is mounted on the axis of rotation of a motor or on a hub which is attached to the axis of rotation of the motor.